package com.itheima.datastructure.demo1;

import java.util.HashSet;
import java.util.Set;

public class LinkCycle {
    static class ListNode {
        int val;
        ListNode next;
        public ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }

    public static void main(String[] args) {
        ListNode node5 = new ListNode(5, null);
        ListNode node4 = new ListNode(4, node5);
        ListNode node3 = new ListNode(3, node4);
        ListNode node2 = new ListNode(2, node3);
        ListNode node1 = new ListNode(1, node2);
        node5.next = node3;
        System.out.println(hasCycle2(node1)); // 优化后的方法
    }

    // 原本使用哈希集合的方法，空间复杂度较高
    public static boolean hasCycle(ListNode head) {
        Set<ListNode> set = new HashSet<>();
        while (head != null) {
            if (!set.add(head)) {
                return true;
            }
            head = head.next;
        }
        return false;
    }

    // 优化后的快慢指针方法
    public static boolean hasCycle2(ListNode head) {
        if (head == null || head.next == null) {
            return false;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (slow != fast) {
            if (fast == null || fast.next == null) {
                return false;
            }
            slow = slow.next;    // 慢指针走一步
            fast = fast.next.next; // 快指针走两步
        }
        return true;
    }
}